1. Field of the Invention
A grating irradiation type moiré fringe measuring and analyzing method has a moiré grating disposed between a light source and a surface to be inspected. The surface is irradiated with light from the light source so as to project a shadow of the moiré grating onto the surface, and an image of moiré fringes formed by the shadow and moiré grating is captured, so as to analyze the surface shape. The present invention relates to a moiré grating noise eliminating method for eliminating a noise component which occurs when an image of the moiré grating itself is captured in addition to that of moiré fringes.
2. Description of the Prior Art
The grating irradiation type moiré fringe measuring and analyzing method has been in use for measuring and analyzing shapes of various surfaces to be inspected, since it can directly form contour images of object surfaces in a noncontact fashion.
In the grating irradiation type moiré fringe measuring and analyzing method, a moiré grating (also referred to as “reference grating”) in which a number of linear grating lines are formed in parallel with each other with a predetermined fine pitch is employed, and irradiation light from the light source projects an image of the moiré grating onto the surface to be inspected. When the surface to be inspected has irregularities, the projected moiré grating shadow deforms according to the form of the surface as seen from a direction different from the light irradiating direction, thus becoming a deformed grating, whereby contour moiré fringes indicating the shape of the surface in terms of overlaps between the deformed grating and moiré grating is observed.
For analyzing the shape of a surface to be inspected with a higher accuracy according to thus obtained moiré fringes, there has recently been known a fringe measuring and analyzing method using a grating irradiation type fringe scanning moiré apparatus which captures a plurality of moiré fringe images while carrying out fringe scanning for shifting the phase of moiré fringes, and analyzes changes in thus captured moiré fringe images by using a computer.
In such moiré fringe analysis, an image of the moiré grating itself may be captured in addition to necessary moiré fringes, thus lowering the accuracy in measurement.
When a region of 25 mm×25 mm is captured by a CCD having 512×512 pixels while using a moiré grating with a grating pitch of 200 μm, for example, each grating line corresponds to 2 pixels, thereby forming an image. This formed grating image may have noise at the time of image analysis, thereby causing analysis errors.
For eliminating such moiré grating noise, there has been known a method in which a moiré grating is moved (shifted) in a direction perpendicular to each grating line within the plane of the moiré grating, and images are captured during shifting, so as to average images of the moiré grating itself. However, conventional methods in which images are simply captured while shifting the moiré grating without fully taking account of the moiré grating shifting speed or moiré fringe capturing timing have been problematic in that they may not fully eliminate moiré grating noise, thus causing errors in analysis results. In a fringe measuring and analyzing method using a grating irradiation type fringe scanning moiré apparatus for carrying out fringe measurement and analysis with a higher accuracy in particular, highly accurate analysis may not be carried out unless moiré grating noise can fully be eliminated, whereby the elimination of moiré grating noise may become a serious problem.